The present invention relates to a process for deinking wastepaper printed with heat fusible inks such as laser inks, and more particularly to such a process which does not require that the wastepaper be repulped prior to treatment. The process uses low frequency ultrasound in conjunction with an organic solvent to remove the fused ink particles from the wastepaper.
There are reports in the literature of ink removal by solvent treatment. Likewise, the use of ultrasound in deinking is known. However, the present invention is the first instance where the two separately known treatments have been combined particularly for deinking laser printed paper.
Conventional methods of deinking and reclaiming wastepaper used in the past involve treating the wastepaper with various deinking chemicals. Such methods were reasonably satisfactory and adequate for paper printed with standard inks that were readily removed or saponified with chemicals at elevated temperatures. However, in recent years, methods of deinking involving cooking and the use of chemicals have become increasingly unsatisfactory for a number of reasons.
Ink formulations have become more and more complex and involve the use of a wide variety of synthetic resins and plasticizers with each ink manufacturer having its own special formulation. Furthermore, multicolored printing and advertisements have become more widespread and these involve a variety of new ink formulations that include new pigments, dyes and toners which cannot be removed or dissolved from wastepaper using conventional deinking chemicals. This is particularly true for laser printed paper.
Laser printed paper is one of the most difficult types of wastepaper to deink. Laser printing and xerography are variations of the same process whereby the inks are fused by heat. In the case of laser printing, both heat and pressure are used in the fusion stage. The fusion mechanism and the nature of the binder used in toner formulations makes these papers most difficult to deink.
For such wastepaper, the conventional methods of washing and flotation are not effective. Washing has been found to be useful only for ink particles less than about 25 .mu.m. The fused toner particles are much larger than this, on the order of about 40 to 400 .mu.m. Screening is also ineffective for the toner particles used in laser printing because of their plate-like shape. Laser printing inks contain a polymer binder and carbon black pigment. The binder is typically a copolymer of styrene and methyl methacrylate (or ethyl methacrylate), with a molecular weight of 70,000 to about 100,000, and a melting temperature in the range of 100.degree.-110.degree. C. The weight ratio of styrene to methacrylate ranges from about 60:40 to 70:30. Since the binder is insoluble in water, aqueous deinking methods can only dislodge the ink particles from the paper fibers. The ink particles must then be agglomerated and separated from the paper fibers by flotation techniques or the like. On the other hand an appropriate solvent treatment may be used to dissolve the binder portion of the laser ink which can then be separated from the paper and collected for disposal. In the case of the present invention, a low frequency ultrasound is used to separate the toner particles from the paper.